ASME PRESSURE VESSELS
The scope of this presentation is to present basic information and understanding of the ASME code for the design of pressure vessels for the chemical and process industry as applicable in the United States and most of North and South America.
RUPTURE HAZARD OF PRESSURE VESSELS
Improperly operated or maintained pressure vessels can fail catastrophically, kill and injure workers and others, and cause extensive damage even if the contents are benign.
Example of Accidents
Three workers were killed and a number of others were injured when a high-pressure vessel containing air and water ruptured. The vessel that ruptured was originally designed with a working pressure of 1740 pounds per square inch (psi), but was operating between 2000-3000 psi. After a number of years of service, the vessel developed a pin-hole leak. The leak was repaired but not in adherence with recognized codes. About a month later, the vessel failed catastrophically at the weld area. The vessel ripped apart and rocketed through the roof. Major pieces of shrapnel weighed from 1000 to 5000 pounds. Some pieces were thrown a half mile away. Fortunately, people on a nearby highway and a nearby commuter railway narrowly missed injury. Damage to the plant was extensive and a portion of the state was without phone and electrical services for many hours.
This accident demonstrates the potential danger of pressure vessels if they are not properly designed, constructed, operated, inspected, tested, or repaired. The higher the operating pressure and the larger the vessel, the more energy will be released in a rupture and the worse the consequences. It should be emphasized that the danger exists even if the vessel contents are not flammable, reactive, or explosive. In the case above, a vessel containing only water and air ruptured and released great energy. Had the contents of the vessel been flammable and/or toxic, the consequences would probably have been magnified.
Factors in Pressure Vessel Failure
The following conditions and factors have played major roles in pressure vessel accidents:
- Operation above the maximum allowable working and test pressures.
- Improper sizing or pressure setting of relief devices.
- Improper operation of relief devices due to faulty maintenance and failure to test regularly.
- Failure of the vessel due to fatigue from repeated pressurization, general thinning from corrosion or erosion, localized corrosion, stress corrosion cracking, embrittlement, holes and leaks.
- Failure to inspect frequently enough.
- Improper repair of a leak or other defect involving welding and annealing that embrittles and further weakens the vessel. Hazards posed by a vessel can be worse if repair welds are made without shutting down and de-inventorying the vessel. If a pressure vessel is repaired without removing the water, the quench effect of the water can embrittle the steel.
- Over pressuring and failure of the vessel due to exothermic reaction or polymerization.
- Vessel exposure to fire.
Pressure Vessel Laws
Requirements for pressure vessels vary widely from state to state. Many states have a boiler law, but others do not. Even for those states that have a boiler law, typical practices (e.g., inspector requirements) for pressure vessels may vary. State boiler laws that require general adherence to American Society of Mechanical Engineers (ASME) codes or National Board Inspection Code (NBIC) usually require the following for each pressure vessel:
- Registering with the state boiler and pressure vessel department.
- Designing and constructing in accordance with Section VIII of the ASME Boiler and Pressure Vessel Code (ASME Code), Rules for Construction of Pressure Vessels, Division 1, which covers vessels operating between 15 psi and 3000 psi.
- Marking the ASME Code on the vessel with specified information that includes the manufacturer, the serial number, the year built, and the maximum allowable working pressure for a specific temperature, and any special suitability such as for low temperature and poisonous gases or liquids.
- Having the vessel approved for installation with the submission of drawings, specifications, welding details and calculations, and having an authorized inspector be satisfied with the welding and witness the testing.
- Operating at pressures below the maximum allowable working pressure with pressure relieving devices set according to the ASME Code; testing at regular intervals.
- Periodically inspecting for corrosion and defects, and testing according to the NBIC Manual for Boiler and Pressure Vessel Inspectors or American Petroleum Institute (API) 510, "Pressure Vessel Inspection Code," for vessels in the petrochemical industry.
- Repairing or altering only according to a plan approved by an authorized inspector and conducted by test-qualified welders. The inspector must be satisfied that the repairs are performed according to NBIC or API 510 and specify any necessary nondestructive and pressure testing. Increasing the maximum allowable working pressure or temperature is considered an alteration whether or not physical work is done.
In states with no pressure vessel law, good safety practices require that similar precautions be followed in the design, construction, welding, testing, marking, operation, inspection, and repair of any pressure vessel. The ASME Code should be used for the design, construction, initial testing, and operation of pressure vessels. The NBIC or API 510 should be used for maintenance and inspection and subsequent testing. Boiler and machinery insurance companies, some pressure vessel suppliers, or jurisdiction-licensed independent contractors can provide authorized inspectors.
Evaluating Potential Explosion Hazard
Facilities, particularly those without formal pressure vessel inspection programs, should survey their vessels, review pertinent history and data to identify hazards, and prevent vessel rupture or catastrophic failure.
Among the questions to be asked and answered are the following:
- Does the vessel operate above 15 psi, and was it designed, fabricated, and constructed according to the ASME Code or other applicable code? Is the vessel code labeled or stamped? Is the operating pressure and size of the vessel known?
- Is the vessel maintained, inspected, and repaired according to the NBIC and/or API 510?
- Are the ratings and settings of the relieving devices appropriate? Are the devices tested regularly and how recently?
- Is the vessel inspected periodically? What are the criteria for inspection frequency? When was it last inspected externally? When was it last inspected internally? Did the inspection disclose general thinning of walls due to corrosion, localized corrosion, stress corrosion cracking, embrittlement, holes, leaks, or any other defects that required follow up? Were they followed up?
- Has the vessel been repaired? Were the plan of repair, welding techniques and safety tests approved by a certified or authorized inspector? Was the welding done by a qualified welder? Were the welding performance qualification tests approved by an inspector? Was the vessel tested after the repair was completed?
- Was the vessel down rated and were the necessary changes in operating conditions and relief device settings made?
- Are exothermic reactions carried out in the vessel? Does the vessel have an emergency relief system to handle runaway reactions?
Case History: Faulty Welds Caused Pressure Vessel Explosion and Fire
This incident provides information regarding an explosion and fire that occurred at the Marcus Oil facility in Houston, Texas in December 2004. Investigators determined that the explosion resulted from faulty welds in a steel process pressure vessel.
In its final investigation report on the explosion, the U.S. Chemical Safety Board (CSB) describes the violent explosion of a 50,000-pound steel pressure vessel at the Marcus Oil and Chemical facility. The explosion was felt over a wide area in Houston and ignited a fire that burned for seven hours. Several residents were cut by flying glass.
Building and car windows were shattered, and nearby buildings experienced significant structural and interior damage.
The Marcus Oil facility refines polyethylene waxes for industrial use. The crude waxes, which are obtained as a byproduct from the petrochemical industry, contain flammable hydrocarbons such as hexane. The waxes are processed and purified inside a variety of steel process vessels. The vessel that exploded was a horizontal tank 12 feet in diameter, 50 feet long, and operated at a pressure of approximately 67 pounds per square inch.
The case study report and accompanying safety recommendations have been posted to the CSB web site (http://www.csb.gov).
Investigators determined that the failed vessel, known as Tank 7, had been modified by Marcus Oil to install internal heating coils, as were several other pressure vessels at the facility. Following coil installation, each vessel was resealed by welding a steel plate over the 2- foot-diameter temporary opening. The repair welds did not meet accepted industry quality standards for pressure vessels. Marcus Oil did not use a qualified welder or proper welding procedure to reseal the vessels and did not pressure-test the vessels after the welding was completed.
Recovered patch plate weld from failed Tank 7
The weld used to close the temporary opening on Tank 7 failed during the incident because the repair weld (see figure) did not meet generally accepted industry quality standards for pressure vessel fabrication. The original, flame-cut surface was not ground off the plate edges before the joint was re-welded, and the weld did not penetrate the full thickness of the vessel head. Furthermore, the welds contained excessive porosity (holes from gas bubbles in the weld). These defects significantly degraded the strength of the weld.
Design Issues – Relief Valves
Investigators found that Tanks 5, 6, 7, and 8, the nitrogen storage vessels, and the compressed-air storage vessel were not equipped with pressure-relief devices, as required by the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code. However, this was not a factor in causing the incident.
Marcus Oil installed a connection between the nitrogen and compressed-air systems to provide rapid pressurization of the nitrogen system when the nitrogen pressure was too low to move molten wax from the tanks to the process unit. The company assumed that compressed air was an acceptable substitute for nitrogen during processing. However, investigators determined that management did not evaluate the hazards that resulted from this process change. Pressurizing the nitrogen system with compressed air contaminated the nitrogen gas with as much as 18 percent oxygen — a level sufficient to support combustion.
Marcus Oil used air instead of nitrogen to boost the pressure of the vessel, and the oxygen inside the tank allowed the ignition of the flammable material, most likely by sparks from the metal fragments. The fire spread back into the damaged tank and caused a violent explosion, which propelled the 25- ton vessel more than 150 feet.
Pressure Vessel Codes
The ASME Boiler and Pressure Vessel Code provides rules for pressure vessel design, fabrication, weld procedures, welder qualifications, and pressure testing. In addition, the National Board of Boiler and Pressure Vessel Inspectors has established the National Board Inspection Code for pressure vessel repairs and alterations. The code requires alterations to pressure vessels to be inspected, tested, certified, and stamped.
"If the provisions of internationally recognized pressure vessel safety codes had been required and enforced, this accident would almost certainly not have occurred," CSB Board Member John S. Bresland said.
The incident at the Marcus Oil facility underscores the importance of compliance with pressure vessel and inspection codes and the use of qualified welders. Equally important is understanding the potential hazards introduced with process changes.
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